why would my 2007 chevy uplander with 3.9 run a little hot?

What is your question?

I have to add about a 1/2 gallon of water every week. but it doesnt seem to be leaking at all. However it does seem to run a little warm and steam comes out of the overflow tank. But the guage says its normal temp. about once a week it runs hot and shuts off the ac to save the motor. that is the only tim the gauge says its hot. any ideas? or ever heard of this happening?